The inverse Monte Carlo (MC) technique, in which transport equations containing coefficients calculated by direct MC methods are solved for the source distribution, will be applied to observations made at depth in a water phantom. The energy/intensity distributions, both radial and azimuthal, of unfiltered 4, 6 and 18 MV photon beams will be obtained from the calculations. More complex equations, containing flattening filter terms in addition to the source terms already derived, will then be solved for the filter profiles by further application of inverse MC; the flattening ability of filters fabricated according to the resulting designs will be tested experimentally. The method will also be applied and experimentally tested in the design of wedge filters and compensators. The objective of the method is to eliminate tedious cut-and-try approaches to beam modification and to increase treatment flexibility by permitting rapid design of beam modifiers to cover a wide range of field sizes, depths, isodose slopes, etc. Increased patient throughput and better accuracy are expected to result. The results will be in a form that could be adapted to computer- controlled fabrication of filters and compensators. Further development of the technique could result in factory production of beam modifiers (and conceivably in targets combining production and flattening functions) eliminating tedious work in the field.